1. Field of the Invention
The present invention relates to a switching power supply device. Particularly, it relates to a switching power supply device in which a semiconductor switch connected to a primary side of a transformer to which a first DC voltage is applied is subjected to switching operation to thereby output a second DC voltage to a secondary side of the transformer.
2. Description of the Background Art
Among insulated switching power supply devices, a flyback type switching power supply device is typically applied to an application for small capacity power not higher than several tens of watts. The flyback type switching power supply device uses a transformer in which windings on a primary side and on a secondary side are opposite to each other in direction or in way of tapping so as to provide reverse characteristics between the primary side and the secondary side. A semiconductor switch connected in series with the primary-side winding of the transformer is subjected to switching operation to transmit primary-side electric power of the transformer to the secondary side thereof. That is, during an ON period of the semiconductor switch, a current flows so that energy can be stored in the transformer. When the semiconductor switch changes over to OFF, the stored energy is outputted from the secondary-side winding of the transformer through a diode.
As a system for stabilizing a secondary-side output voltage of the flyback type switching power supply device, there has been known a secondary-side control system in which fluctuation of the secondary-side output voltage is fed back to a primary-side control circuit by a photocoupler (e.g. see Fairchild Semiconductor, “AN-6756_JA Applying FAN6756 to Control a Flyback Power Supply with Ultra-Low Standby Power”, [online], Mar. 22, 2012, Fairchild Semiconductor Corporation [Searched on Sep. 1, 2014], Internet <URL:https://www.fairchildsemi.co.jp/an/AN/AN-6756.pdf>). Moreover, there has been known a primary-side control system in which an auxiliary winding having the same polarity as that of the secondary-side winding of the transformer is used so that fluctuation of the voltage of the secondary-side winding can be indirectly detected by the auxiliary winding and fed back to a primary-side control circuit (e.g. see JP-A-2013-116026 (Paragraphs [0003] to [0005], FIG. 10)).
In the secondary-side control system, fluctuation of the secondary-side output voltage is detected by a shunt regulator, and the detected fluctuation of the output voltage is fed back to the primary side through the photocoupler to thereby stabilize the output voltage. The switching power supply device using the secondary-side control system however requires lots of components including the shunt regulator and the photocoupler in order to stabilize the output voltage. Therefore, attention has been recently focused on the primary-side control system which can reduce the number of components.
FIG. 7 is a circuit diagram showing an example of a flyback type switching power supply device using a primary-side control system according to the background art. FIG. 8 is a view of operation of the flyback type switching power supply device according to the background art when there occurs a load transient. FIG. 9 is a circuit diagram showing an example of the flyback type switching power supply device having a noise reduction function.
The switching power supply device has terminals 11 and 12 which receive a DC input voltage Vin. A smoothing capacitor C1 is connected to the terminals 11 and 12. A series connection circuit between a primary winding Np of a transformer T and a semiconductor switch S1 made of an MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is connected in parallel with the capacitor C1. A control circuit 13 which controls the switching power supply device is connected to the semiconductor switch S1.
The transformer T has an auxiliary winding Naux in order to obtain a power supply of the control circuit 13. A rectifying and smoothing circuit constituted by a diode D1 and a capacitor C2 is connected to the auxiliary winding Naux so that an output voltage of the rectifying and smoothing circuit can be supplied to the control circuit 13. A series circuit between resistors R1 and R2 is connected in parallel with the auxiliary winding Naux and a common connection portion between the resistors R1 and R2 is connected to the control circuit 13 to supply a feedback signal to the control circuit 13. Thus, primary-side control is performed.
Configuration is made in such a manner that a rectifying and smoothing circuit constituted by a diode D2 and a capacitor C3 is connected to a secondary winding Ns of the transformer T so that a DC output voltage Vout can be outputted to terminals 14 and 15. A dummy resistor R3 for suppressing the DC output voltage Vout from increasing under a light load or no load is connected to the terminals 14 and 15.
Incidentally, in the switching power supply device, a reference potential of the primary-side circuit is connected to a primary-side ground PGND and a reference potential of the secondary-side circuit is connected to a secondary-side ground SGND.
In the switching power supply device having the aforementioned configuration, when a DC input voltage Vin is supplied to the terminals 11 and 12, the DC input voltage Vin is smoothed by the capacitor C1 and applied to the series circuit between the primary winding Np of the transformer T and the semiconductor switch S1. Here, when the control circuit 13 makes control to turn ON the semiconductor switch S1, a current flows into the primary winding Np of the transformer T so that energy can be stored in the transformer T. Next, when the control circuit 13 makes control to turn OFF the semiconductor switch S1, the energy stored in the transformer T is released so that a current can flow from the secondary winding Ns through the diode D2. This current is smoothed by the capacitor C3 to turn into a DC output voltage Vout.
When a voltage occurs in the secondary winding Ns, a voltage proportional to the voltage (voltage in which a forward voltage of the diode D2 is added to the DC output voltage Vout) occurring in the secondary winding Ns also occurs in the auxiliary winding Naux. This voltage is divided by the resistors R1 and R2. A voltage signal divided by the resistors R1 and R2 is supplied as a feedback signal corresponding to the DC output voltage Vout to a terminal VS1 of the control circuit 13. The control circuit 13 outputs a gate signal Vg for controlling the semiconductor switch S1 to a terminal OUT. On this occasion, the control circuit 13 changes a cycle or an ON-time ratio of the gate signal for turning ON/OFF the semiconductor switch S1 based on the voltage signal supplied to the terminal VS1, to thereby stabilize the DC output voltage Vout. That is, in the case where the DC output voltage Vout substantially rarely changes asunder a light load or no load, the control circuit 13 fixes an ON-width of the gate signal Vg of the semiconductor switch S1 (to a minimum width) and elongates the cycle (reduces the frequency) of the gate signal Vg to thereby reduce power consumption. On the other hand, when the load becomes heavy and the DC output voltage Vout decreases, the control circuit 13 shortens the cycle (increases the frequency) of the gate signal Vg of the semiconductor switch S1 in accordance with the voltage supplied to the terminal VS1. When the frequency of the gate signal Vg of the semiconductor switch S1 reaches the highest frequency in some cases, the control circuit 13 performs PWM (Pulse Width Modulation) control for increasing the ON-time ratio so as to increase the energy supplied to the secondary-side circuit.
Description will be made here about the operation of the switching power supply device in which there occurs a load transient to a heavy load when the switching power supply device is operating under a light load or no load. When the switching power supply device is operating under a light load or no load, a fine current flows as the output current lout and the DC output voltage Vout is controlled to a substantially constant voltage, as shown in FIG. 8. In addition, the semiconductor switch S1 performs switching operation in accordance with a gate signal Vg having a long cycle Ta to thereby supply energy consumed by the fine current.
When the load state suddenly changes from the light-load or no-load state to a heavy-load state, the output current Iout suddenly increases to a value corresponding to the load but the DC output voltage Vout decreases gradually. As soon as the decrease of the DC output voltage Vout caused by the load transient is detected based on a signal applied to the terminal VS1 of the control circuit 13 during next switching operation of the semiconductor switch S1, the control circuit 13 changes the cycle of the gate signal Vg to a short cycle Tb. In this manner, the DC output voltage Vout increases gradually to return to the voltage of a predetermined value. Incidentally, the degree of the decrease (ΔVout) of the DC output voltage Vout changes depending on the timing when the load transient occurred in the cycle Ta in which the switching power supply device was under the light load or no load. That is, as the timing when the load transient occurred is closer to immediately after the pulse output of the gate signal Vg, the degree of the decrease (ΔVout) of the DC output voltage Vout is smaller. As the timing when the load transient occurred is closer to the next pulse output of the gate signal Vg, the degree of the decrease (ΔVout) of the DC output voltage Vout is larger due to occurrence of a delay in the detection of the load transient. Incidentally, when the load suddenly changes from the heavy load to the light load or no load, the gate signal Vg changes from the short cycle (high frequency) to the long cycle (short frequency), causing no delay in the detection of the load transient. Accordingly, the degree of the increase (ΔVout) of the DC output voltage Vout becomes small.
In the switching power supply device, EMI (Electro-Magnetic Interference) noise occurs in accordance with switching operation of the semiconductor switch S1. However, such EMI noise is generally reduced (e.g. see Fairchild Semiconductor, “AN-6756_JA Applying FAN6756 to Control a Flyback Power Supply with Ultra-Low Standby Power”, [online], Mar. 22, 2012, Fairchild Semiconductor Corporation [Searched on Sep. 1, 2014], Internet <URL:https://www.fairchildsemi.co.jp/an/AN/AN-6756.pdf>, or JP-A-6-98539 (Paragraphs [0014] to [0018], FIG. 1)).
A switching power supply device shown in FIG. 9 has such a noise reduction function as described in Fairchild Semiconductor, “AN-6756_JA Applying FAN6756 to Control a Flyback Power Supply with Ultra-Low Standby Power”, [online], Mar. 22, 2012, Fairchild Semiconductor Corporation [Searched on Sep. 1, 2014], Internet <URL:https://www.fairchildsemi.co.jp/an/AN/AN-6756.pdf> or JP-A-6-98539 (Paragraphs [0014] to [0018], FIG. 1). That is, in the switching power supply device, a capacitor C4 is inserted between the primary-side ground PGND of the primary side and the secondary-side ground SGND of the secondary side which are floating to each other in the transformer T. Thus, EMI noise occurring due to switching operation of the semiconductor switch S1 is dropped from the primary-side ground PGND nearest to the source of the EMI noise to secondary-side ground SGND serving as a load-side frame ground through the capacitor C4 so that the EMI noise can be attenuated.
In the flyback type switching power supply device using the primary-side control system according to the background art, the secondary-side output voltage cannot be detected and fed back to the primary side unless the semiconductor switch performs switching operation. Particularly, when the switching power supply device is operating under a light load or no load, the cycle of the switching operation of the semiconductor switch is long. Accordingly, when the load suddenly changes to a heavy load in the long cycle, the load transient cannot be detected until next switching operation. Therefore, detection delay occurs before the decrease of the output voltage caused by the load transient is detected by the next switching operation and the switching operation based on a short cycle is started. There is a problem that the output voltage may decrease largely transitionally at some timing of the load transient.